Möbius–Hückel Topology Switching in Expanded Porphyrins: EPR, ENDOR, and DFT Studies of Doublet and Triplet Open-Shell Systems

The one-sided Möbius band topology with its characteristic 180° twist has fascinated and inspired philosophers, artists and scientists since a long time. On the molecular level, only in the last 13 years a few chemistry groups succeeded to artificially create novel compounds with Möbius symmetry by theory-based molecular design and elaborate chemical synthesis. The interest in molecules with Möbius band symmetry was greatly stimulated in 1964 by a theoretical paper by Edgar Heilbronner from the ETH Zurich. He predicted that sufficiently large [n]annulenes with a closed-shell electron configuration of 4n π-electrons should allow for sufficient π-overlap stabilization to be synthesizable by twisting them into the Möbius topology of their hydrocarbon skeleton. In 2003, the first synthesis of an aromatic Möbius annulene was accomplished by Rainer Herges and co-workers in Kiel. In 2007, Lechosław Latos-Grażyński and co-workers in Wroclaw succeeded in synthesizing free-base di-p-benzi (Yoneda et al., Angew Chem Int Ed 53: 13169–13173, 2014) hexaphyrin (1.1.1.1.1.1), compound 1, an expanded porphyrin which can dynamically switch between Hückel and Möbius conjugation upon changes of solvent and temperature. Shortly thereafter, in 2008, Atsuhiro Osuka and his co-workers from Kyoto, Seoul and Hyogo published the synthesis of an expanded porphyrin in which metalation triggered the production of molecular twisting and Möbius aromaticity. In this minireview, among other studies also our recent EPR, ENDOR and DFT studies on open-shell states of 1, i.e., on the ground-state radical cation doublet state (total electron spin S = 1/2) and the first excited triplet state (S = 1) are summarized. The review is largely based on a previous joint publication of the current authors with the Latos-Grażyński group on radical cations of 1 (Möbius et al., Phys Chem Chem Phys 17:6644–6652, 2015). The radical cation study was the first one of an open-shell π-system with Möbius topology. In the doublet state, the hyperfine interactions of the unpaired electron spin with specific magnetic nuclei in the molecule was used as a sensitive probe for the electronic structure of the molecule and its symmetry properties. This work has now been extended to state-of-the-art DFT theory studies on photo-excited triplet states of 1. In the open-shell triplet state, besides hyperfine couplings, a change of the zero-field splitting interaction between the two unpaired electron spins is predicted to be a viable sensor for electronic structure changes upon Möbius-to-Hückel topology switching.